Presently, large scale application of wind energy in the world is mainly implemented by way of “wind rotor˜generator˜electrical power network˜user”. The electrical power network acts as load of wind power and power supply for user, and ensures the utilization of wind power. Wind power connected to power system is an exclusive application of large scale wind power station in the world. However, the electric current generated by wind power has the characteristic of random great fluctuation due to the high instability of wind energy. Thereby, the ratio of wind power in the electrical power network is restricted seriously so that the usage percentage of wind power in the terminal load is difficult to go beyond 10%, which is a difficult problem in the world. At the same time, for satisfying the critical requirements of stable frequency and stable phase, grid-connected wind power utilizes the latest related technology achievements and materials in the world, which results in the complication of equipment and technique and reduces the usage efficiency of wind power. Thereby, the cost of wind power is greatly raised and the large scale development of wind power industry is restricted seriously.
Wind speed in wind field changes greatly. For a wind turbine rotor with fixed pitch and constant rotation speed, attack angle of airflow with respect to blade airfoil of wind rotor increases with the increase of wind speed in the wind field. When the wind speed reaches a set value, flow separation occurs on the upper surface of blade and thereby blade stall occurs. With the further increase of the wind speed, there is a change from shallow stall to deep stall. FIG. 1 shows foil speed and torque coefficient corresponding to different wind speed, wherein portion 1-1 shows the speeds in deep stall state at point m of the curve in FIG. 1, portion 1-2 shows the speeds in shallow stall state at point n of the curve, portion 1-3 shows the speeds in non-flow-separation state at point p of the curve, where “c” in portion 1-1, 1-2 and 1-3 represents axial speed related to the wind speed, “u” represents tangent speed related to rotational speed of the wind rotor, and “w” represents relative speed related to the blade of the wind rotor. Due to the operation conditions of the wind turbine rotor such as fixed pitch and constant rotation speed, the stable operation limits of the wind turbine are small. In order to increase effective operation limits and fully meet the technique requirements, the wind turbine rotor with adjustable pitch and variable rotation speed but more complicated structure is employed. With the increase of the wind speed, the wind turbine can work in wider operation limits such that the pitch of the blades can be adjusted by the pitch regulation to adapt the change of flow direction and avoid blade stall, and such that the operation efficiency is higher by speed change. However, the adjustable-pitch regulation mechanism and speed-change system increase the complexity of the wind turbine and thereby greatly increase the cost and maintenance work amount.
Non-grid-connected power generation involves non-grid-connected wind power. Wind power is directly used for the users, which helps to utilize and fully develop wind power. Thereby, not only the attack of wind power to the electrical power network is avoided, but also the unnecessary cost due to the severe conditions for synchronization of wind power to utility power system is decreased. Only greatly decreasing the cost of wind power can attract high power-consumption users to use wind power. Therefore, for breaking through the bottle-neck of development of wind power, it is an extremely effective measure to vigorously develop non-grid-connected wind power. Upon many years of investigation and experiments, since high power-consumption users such as electrolytic aluminum industry, electrolytic copper industry, electrolytic manganese industry, salty water electrolyzing industry, sea-water desalination industry mainly utilize the direct current to make large-scale production, they may directly utilize wind power. However, the present price of wind power is still high. Therefore, for greatly decreasing the cost of wind power and thereby accelerating the rapid development of the wind power industry, it is necessary to develop a wind turbine generator system with high efficiency, high reliability and low cost.